An anaerobic digester, alternatively called a fermenter or biogas plant, may have a tank in which a substrate is digested, or fermented, and biogas is produced. The tank is typically closed at the top with a cover. The tank contains a mixture of solids and liquids, also referred to as the digestate, liquid contents, fermentation section, or sludge, which is being digested and producing biogas. Biogas collects in the head space of the tank, alternatively called a gas section, above the liquid contents. An agitator or mixer is typically provided within the tank to agitate, stir, mix, or at least partially homogenize the liquid contents to encourage biogas production and digestion.
The agitators, however, may require inspection, maintenance or repair at some times. Agitators may be supported on a generally vertical holding rod or otherwise made movable between the liquid contents and the gas section. However, access to the agitator still needs to be provided through the cover.
German patent application DE 199 51 959 A1 describes a digester having a flat concrete cover. A rectangular maintenance shaft extends downwards from the cover into the liquids portion. An agitator can be lifted into the maintenance shaft. A hatch through the cover allows access to the agitator in the maintenance shaft.
European Patent Number EP 1 717 305 B1 describes a service pit that is mounted on a frame connected to the inside wall of a digester tank. The sides of the service pit are sealed to a dome shaped tank cover. An agitator can be moved upwards into the service pit for servicing. Part of the service pit extends upwards above the cover. A vertical door in the front of the service pit above the cover can be opened to access the agitator. International Publication Number WO 2010/084003 describes a variation of this design in which the service pit is mounted to a frame connected to the outside of the tank, to an extension of the wall of the tank, or to the ground outside of the tank.
European Patent Number 2 270 128 B1 describes a servicing device for a digester with a movable sealing device. The sealing device has a gas retention wall made of a flexible material. The sealing device can be raised into an inoperative position or lowered into a sealing position. In the inoperative position, the sealing device does not interfere with the operation of the digester. In the sealing position, the lower edge of the sealing device is located on or in the liquid contents of the digester and separates the inside of the servicing device from the gas section of the digester. While the sealing device is in the sealing position, an agitator can be moved upwards into the servicing device. A door in the servicing device is opened and, after the biogas escapes from the servicing device, the agitator can be serviced.